1. Field of the Invention
The invention relates to systems production of multiphase petroleum effluents situated offshore at great water depths.
2. Description of the Prior Art
Many petroleum production systems are described in the prior art.
French Patent 2,665,725 describes a multiphase production system suited for reservoirs with low production capacities, that are situated in moderately deep or shallow waters. The concept of this system is based on the use of a floating structure, readily movable and provided with the necessary equipments for transferring effluents from a well to the floating structure prior to sending them to a processing or storage site. Transfer of these effluents is performed without separation of their constituents. The anchoring ties of this buoy are flexible enough to allow to displace it readily from one reservoir to another.
In patent application PCT-NO97/00,068, a submersible floating buoy is anchored above a reservoir comprising several production wells. Anchoring of this buoy is achieved by means of production risers that extend between the floating buoy and the developed area of the reservoir. Production is fed into the production risers and carried to this submersible floating structure, then sent to a processing and production plant, floating or not, such as a converted tanker, or FPSO, where it is collected and processed in order to be carried to a point of destination and of use.
Although such a system decreases manufacturing cost by saving installing equipments on the sea bottom and by using the production risers as anchors, it however has certain drawbacks. In fact, the FPSO is suitable when the developed fields have low production capacities, but it becomes less profitable in the opposite case. Furthermore, although the buoy is situated at a depth selected to minimize the effects of the sea currents and of the wave motion, the influence of these two parameters produces relative motions of the FPSO in relation to the floating structure that can lead to stresses in the line used to transfer the effluents to the FPSO. Under certain conditions, such stresses can even result in breakage of these lines.
Moreover, in the case where the pressure of the reservoirs is low, bringing the effluents up to the surface can be difficult for great water depths, or even impossible if the reservoir pressure is insufficient, and the aforementioned system is ill-suited.